Copper alloy exhibiting gamma alumina surface and method



United States Patent 3,551,214 COPPER ALLOY EXHIBITING GAMMA ALUMINA SURFACE AND METHOD James A. Ford, North Haven, Elmer J. Caule, New

Haven, and Philip R. Sperry, North Haven, Conn., assignors to Olin Corporation, a corporation of Virginia No Drawing. Filed Jan. 29, 1968, Ser. No. 701,062 Int. Cl. C23f 7/02; C22c 9/00 US. Cl. 148-631 14 Claims ABSTRACT OF THE DISCLOSURE A tarnish resistant alloy consisting essentially of from 1.0 to 9.5% aluminum, up to 5.5% silicon, 0.01 to 0.5% of either phorphorus, arsenic, selenium and bismuth, and at least one element from the group of from 0.05 to 1.5% Zinc, 0.05 to 0.3% cadmium, 0.05 to 0.3% beryllium, and 0.05 to 0.5% magnesium, balance copper, and the method of producing the alloy.

DISCLOSURE It has been disclosed in US. Pat. No. 3,259,491 that proper combinations of ternary alloying additions to Cu, which were selected on the basis of being capable of forming a complex stable oxide, of having sufiicient solid solubility in Cu to be available for diffusion under proper conditions to the metal surface and being balanced to provide diffusion to the surface in proper proportions to form the stable oxide, will provide unusual oxidation resistance.

It has further been shown in US. Pat. 3,341,369 that the class of copper base alloys containing combinations of Group III with Group IV alloying additions, not only show good elevated temperature oxidation resistance, but also provide enhanced oxidation resistance at lower temperatures in the metal surface beneath the original continuous oxide film. These alloys when heated in an oxidizing atmosphere at high temperatures, e.g. 750 C., form a transparent oxide film which both preserves the original color and luster of the alloy, and also provides the oxidation and improved tarnish resistance.

Also, in U.S. application S.N. 584,097, filed Oct. 4, 1966, Group V elements including phosphorus, arsenic, antimony, and bismuth have been found desirable for enhanced tarnish resistance in the alumina film formed at elevated temperature, e.g. 450 C. to 800 C.

As is also disclosed in said Ser. No. 584,097, the previously mentioned Group V elements are equally effective in an alloy containing cobalt containing 0.05 to cobalt, e.g. Cu3A12Si-0.1P0. 3Co.

Although these alloys do afford excellent oxidation resistance at elevated temperatures, they do not form protective films at lower temperatures down to, and including, ambient.

It is an object of the present invention to extend the range of temperature toward ambient over which a highly protective oxide film is formed.

It is another object of the present invention to provide a tarnish resistant alloy.

The protective oxide film formed on the foregoing alloys has been identified as gamma alumina. It is known Patented Dec. 29, 1970 that gamma alumina has a defect spinel structure and that suitable alloying will provide changes in its resistance to ionic movement under a potential. Experience gained with additions to Al alloys has demonstrated that, for example, a small addition of Mg (0.1% will increase both the ionicand electronic resistance of the alumina film and limit its growth. However, larger additions reverse this behavior by promoting the formation of MgO, rather than alumina.

Based .on these considerations, the present invention embodies additions which, when present in the alloy solid solution in small amounts, will promote the formation of more perfect oxide films, with resulting enhanced oxidation and tarnish resistance. In the same sense that small amounts of specific impurities added to semi-conducting compounds will have large effects on their conductive properties, these minor additions can be considered as dopants for the oxide formed on the metal surface.

In order to achieve the desired improved oxide film, specifically an alumina film on the Cu-Al-Si Group V alloy, small additions of Group II elements Zn, Cd, Be and Mg have been made. The Group II additions may be provided in the following compositional ranges to the basic alloy:

combination with the Group II elements is 1.0 to 9.5 Al, and up to 5.5 Si. Furthermore, the optimum alloy composition has been established at 2 to 4% Al and 1 to 3% Si.

It has also been found that the previously mentioned Group V elements (P, As, Sb, Bi) can be used in combination with the Group II elements in the range of 0.01 to 0.5%. However, the preferred Group V element is phosphorous.

Cobalt also may be used in the range of 0.05 to 5%.

These alloys may be thermally oxidized over the temperature range of 300550 C., from 5 minutes to 100 hours in an oxidizing atmosphere. Even longer oxidizing times could be utilized, if desired, but in general are not justifiable economically.

The following examples illustrate the invention without limiting its scope.

EXAMPLES The table below contains data obtained from oxidizing copper base alloys containing 3% aluminum, 2% silicon, and 0.1% phosphorus at various temperatures. The particular alloy additions are given in the left-hand column. At each of the temperatures specified, the alloys were oxidized for a period of time of two hours. A capacitance bridge technique was used for measuring the film resistance reported. See, for example, Becket al., International Journal of Corrosion Science, vol. 2, pp. 133-145, 1962.

The results in the table show that the cadmium, beryllium, zinc, and magnesium all provide tarnish resistance after exposure to the outdoors (roof exposure racks) for respectively, 10 and days, provided that the alloys are oxidized within a temperature range of from 350 to 550 C.

References Cited UNITED STATES PATENTS Morris 75-162 Hensel et a1 75162X Pryor 75-162 Capula et a]. 75-153X FOREIGN PATENTS 138,568 1934 Austria 75-162 519,902 1940 Great Britain 75162 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

